1. Field of the Invention
The present invention relates to frequency selective limiters (FSLs) for providing low attenuation to relatively low power RF signals and high attenuation to relatively high power signals above a critical threshold level and more particularly to a magnetostatic wave (MSW) type frequency selective limiter having a threshold level in the range of −75 dBm to −35 dBm.
2. Description of Related Art
In an RF environment, interference signals may have jamming effects on an RF receiving system even though they are not intentionally generated. Jamming interference signals may originate, for example, in a laptop computer on an airplane, a radio or television tower, or any high frequency device such as radar, a radio or cellular telephone. Also jamming signals may be generated by combatants in a military environment or by terrorists in a similar or commercial environment.
Frequency selective limiters (FSLs), also known as power selective limiters (PSLs), are well known in the art for suppressing RF signals in various types of equipment having a need for protection against interference and jamming. In one type of frequency selective limiter, known as a magnetostatic wave limiter, magnetostatic waves propagate in a film of magnetically biased ferrite material such as yttrium iron garnet (YIG) which is grown epitaxially on a substrate. In a magnetostatic surface wave (MSW) device a permanent magnet bias field is oriented perpendicularly to the direction of the wave propagation in the plane of the film wherein energy concentrations are bound to the top surface of the epitaxial film in a forward going wave mode and to the bottom surface of the film at a single reverse going wave mode. This principle is well documented in the art.
Frequency selective limiters are known to have particular applicability in radio frequency systems used, for example, for navigational purposes and, more particularly, to receivers which operate in satellite navigation systems such as the well known Global Positioning System (GPS). One such example is disclosed in U.S. Pat. No. 5,955,987, entitled, “Hybrid Radio Frequency System With Distributed Anti-Jam Capabilities For Navigation Use”, issued to John H. Murphy et al. on Sep. 21, 1999, and which is assigned to the assignee of the present invention. The contents of this patent are herein incorporated in their entirety by reference for any and all purposes.
The Murphy et al. patent discloses, among other things, an adaptive RF filter based on YIG filter technology and discloses a magnetostatic wave (MSW) structure as shown in FIG. 1 of this application. As shown in FIG. 1, a thin film of yttrium iron garnet (YIG) 12 is located transversely across a pair of input and output stripline conductors 14 and 16 which are known in the art as transducers. When appropriately biased by an external magnetic field 18, the structure functions as a signal delay line having a variable impedance which utilizes a spin wave to absorb excess energy above a threshold value from signals applied to the input transducer, i.e., the stripline conductor 14. The threshold is tunable and is a function of the thickness dimension (a) of the YIG film 12. The frequency selective device 10 operates as a frequency or, more precisely, a power selective limiter and has found application not only in GPS systems where jammer-free and interference-free operation is required with respect to an RF receiver, but also on other types of airborne vehicles as well as surface vehicles or other platforms for the computation of the platform position.
GPS systems typically include a frequency selective limiter (FSL) 20, as shown in FIG. 2 between the front end of a GPS receiver 22 and a receiving antenna 24 which may or may not also include a low noise RF amplifier 26. Present FSLs have a threshold power level of approximately −25 dBm. However, because of the extremely low power levels of the received GPS signal, significant gain (>60 dB) is required to bring the GPS signal plus interference into a power range where the large interfering signal can be selectively attenuated. The amplifiers require significant power and the high gain can lead to instability in compact/integrated designs. FSLs with threshold power levels in the −75 dBm to −35 dBm range would eliminate or significantly reduce the required amplifier gain and power.